The present invention relates generally to the field of magnetic recording media of the type used in rigid disk drives for computer data storage, and more particularly to magnetic alloys used in such magnetic recording media which include, inter alia, a relatively large percentage of nitrogen to provide high coercivity and saturation magnetization, and low noise and PW50.
Recording performance for magnetic disks are determined by three basic characteristicsxe2x80x94narrow PW50, high overwrite, and low noise. PW50 is the pulse width of the bits expressed in either time or distance, defined as the width of the pulse at half-maximum. Having a narrower (and more well-defined) pulse allows for higher recording density. A wide PW50 means that the bits are crowded together, causing them to interfere with each other. This interference is termed inter-symbol interference. Excessive inter-symbol interference limits the packing density of bits in a given area.
Conventionally, there are number of media factors which affect PW50. In order to achieve narrow PW50, the coercivity (xe2x80x9cHcxe2x80x9d) of the media must be high. However, if Hc is too high, the head field will have a difficult time saturating the media, resulting in poor overwrite. Overwrite (xe2x80x9cOWxe2x80x9d) is a measure of the ability of the media to accommodate overwriting existing data. That is, OW is a measure of what remains of a first signal after a second signal (for example of a different frequency) has been written over it on the media. OW is poor when a significant amount of the first signal remains. OW is generally affected by Hc, thickness, and the hysteresis loop squareness of the film.
PW50 may be reduced by using a thinner magnetic film. Another means of reducing PW50 is to increase hysteresis loop squareness, and narrow the switching field distribution, as described by William and Comstock in xe2x80x9cAn Analytical Model of the Write Process in Digital Magnetic Recording,xe2x80x9d A.I.P. Conf. Proc. Mag. Materials, 5, p. 738 (1971). Hysteresis loop squareness (xe2x80x9cSxe2x80x9d) has several components, including coercivity squareness (xe2x80x9cS*xe2x80x9d) and remnant coercivity squareness (xe2x80x9cS*remxe2x80x9d).
Noise performance of a magnetic film can be defined in terms of read jitter and write jitter. In peak-detection type recording channels. Noise, together with inter-symbol interference, contributes to the uncertainty in the location of the individual bits, which cause the data to be read with some displacement in timing from that which is expected. This displacement is referred to as bit shift. The bit shift needs to be reduced to a minimum for a given timing window of the bit in order to assure accuracy in reading the bit.
Read jitter is primarily determined by the amount of signal available from the bit, and the electronic noise in the channel. A thicker magnetic film will typically provide reduced read jitter. Unlike read jitter, write jitter is determined by the intrinsic noise of the film. Intrinsic media noise has been theoretically modeled by Zhu et al. in xe2x80x9cMicromagnetic Studies of Thin Metallic Filmsxe2x80x9d, J. Appl. Phys., vol. 63, no. 8, p. 3248 (1988), which is incorporated by reference herein. Chen et al. describe the source of intrinsic media noise in xe2x80x9cPhysical Origin of Limits in the Performance of Thin-Film Longitudinal Recording Media,xe2x80x9d IEEE Trans. Mag., vol. 24, no. 6, p. 2700 (1988), which is also incorporated by reference herein. The primary source of intrinsic noise in thin film media is from the interparticle exchange interaction. In general, the higher the exchange interaction, the greater the noise.
The noise from interparticle exchange interaction can be reduced by isolating the individual particles. This may be accomplished by spacing the grains apart from one another, or by interposing a non-magnetic material or insulator at the grain boundaries as described by Chen et al, in the aforementioned xe2x80x9cPhysical Origin of Limits in the Performance of Thin-Film Longitudinal Recording Media.xe2x80x9d The amount of separation needs to be only a few angstroms. There is another interparticle interaction, called magnetostatic interaction, which acts over a much greater distance between particles as compared to the exchange interaction. Reducing the magnetostatic interaction does reduce intrinsic media noise slightly. However, the effects of magnetostatic interaction actually improve hysteresis loop squareness and narrow the switching field distribution, and hence improve PW50 and OW Therefore, magnetostatic interaction is generally tolerated.
In order to obtain the best performance from the magnetic media, each of the above criteriaxe2x80x94PW50, noise and OWxe2x80x94must be optimized. This is a formidable task, as each of these performance criteria are inter-related. For example, obtaining a narrower PW50 by increasing the Hc will adversely affect OW, since increasing Hc degrades OW. A thinner media having a lower remnant magnetization-thickness product (xe2x80x9cMrTxe2x80x9d) yields a narrower PW50, however the read jitter increases because the media signal is reduced. Increasing squareness of the hysteresis loop contributes to narrower PW50, but generally increasing squareness increases noise. Thus, the amount that PW50 may be narrowed is limited by the increase in noise. Providing a mechanism for separating or isolating the grains to break the exchange coupling can effectively reduce the intrinsic media noise. Noise is improved by eliminating the interparticle exchange interaction. A slight further reduction of noise is possible by reducing magnetostatic interaction, but this reduces the hysteresis loop squareness and increases the switching field distribution, which degrades PW50 and OW.
In order to obtain the optimum media performance, the MrT must be reduced for between OW and PW50, but still retain sufficient signal to maintain acceptable read jitter. This is principally accomplished by reducing the film thickness (thereby reducing the space loss between the recording head pole tip and the media), and using an alloy having a higher saturation magnetization (xe2x80x9cMsxe2x80x9d).
Therefore, an optimal thin film magnetic recording media for high density recording applications, i.e., that can support high bit densities, requires low noise without sacrificing the switching field distribution. S*, and S*rem. Recording density can then be increased since bit jitter is reduced. In order to achieve the best compromise in performance, the individual grains of the magnetic film must be isolated to eliminate the exchange interaction, and grains must be uniform and have a tight distribution of sizes to minimize intrinsic media noise while maintaining high hysteresis squareness.
One type of magnetic media which has allowed optimizing certain of the above performance criteria is based on alloys of cobalt (Co) and platinum (Pt). CoPt is typically alloyed with nickel (Ni), chromium (Cr), etc. Attributes of CoPt alloys have been described by Murdock et al. in xe2x80x9cRoadmap to 10 Gb/in2 Media: Challengesxe2x80x9d, IEEE Trans. Mag., 1992, page 3078, by Opfer et al. in xe2x80x9cThin Film Memory Disk Developmentxe2x80x9d, Hewlett-Packard Journal (Nov. 1985), and by Aboaf et al., in xe2x80x9cMagnetic Properties and Structure of Co-Pt Thin Filmxe2x80x9d, IEEE Trans. Mag., page 1514 (1983), each incorporated herein by reference. Increasing storage capacity demands and performance requirements have motivated a search for ways to improve Co-Pt based alloys.
As stated above, a high Hc film will produce a narrow PW50, thus permitting an increase in storage density. One method of increasing Hc involves increasing the atomic percent (xe2x80x9cat. %xe2x80x9d) of platinum in the film. This approach is described in Opfer et al., xe2x80x9cThin-Film Memory Disk Developmentxe2x80x9d (referred to above). However, it is known that as the platinum content increases, SNR decreases due to a reduction in signal amplitude without a commensurate decrease in media noise.
In order to decrease the media noise, it is also known to introduce oxygen into the magnetic film in a concentration of 5 to 30 at. %, as taught by Howard et al. in U.S. Pat. No. 5,066,552, incorporated by reference herein. However, as pointed out by Howard et al. in said patent, introducing oxygen decreases both Hc and S*.
Another approach to increasing Hc, as discussed by Maeda in xe2x80x9cEffects of Nitrogen on the High Coercivity and Microstructures of Co-Ni alloy films,xe2x80x9d Journal of App. Phys., vol. 53, no. 10, pp. 6941-6945 (October 1982), involves depositing a thin film magnetic media by sputtering in an ambient of argon and nitrogen. An increase in the nitrogen gas concentration in the chamber (e.g., about 24% by volume) is shown in this reference to be accompanied by an increase in Hc. However, magnetic films produced by this method exhibit a decrease in Ms. Also, the film produced by this method is not ferromagnetic as deposited. This method requires the additional step of annealing the deposited film at a relatively high temperature to diffuse large amounts of the nitrogen out of the cobalt film, thus rendering the film ferromagnetic.
Current and future demands of high-density magnetic media are foreclosing the opportunity for a trade off between Hc, Ms, SNR, etc. Therefore, there is at present a need in the art for a method of increasing the coercivity of a thin film magnetic alloy while yielding a high degree of squareness, high SNR, high overwrite, and low PW50.
The present invention is an improved CoPt based alloy doped with in excess of 1 atomic percent nitrogen in the film in order to increase the signal-to-noise ratio of the alloy, and a method for forming a magnetic recording media including this alloy. The alloy may be doped up to the limit at which coercivity falls below a preset target value and squareness falls to an undesirable level. A magnetic alloy is disclosed which has been doped with nitrogen and has a high coercivity, for example in the range of 1400 Oe or above, and simultaneously an increased SNR as compared to the same alloy which has not been doped with nitrogen. High squareness is also obtained as compared to the undoped alloy.
We believe that nitrogen is introduced such that inter-grain spacing is minimized and gain growth is very uniform. The effects of nitrogen are greatly enhanced by employing a nucleation layer under the magnetic layer. The nucleation layer typically contains NiP, and may be doped for example with an oxide. One specific nucleation layer is comprised of Ni3P together with 1 wt. % of Al2O3. Importantly, the grain structure of the nucleation layer and nitrogen-containing magnetic alloy facilitate nitrogen separation in the interstices between the grains of the magnetic film, thereby reducing noise caused by exchange interaction between grains. Grain xe2x80x9cclusteringxe2x80x9d is minimized, and uniform grain topology is achieved. We have found that the benefits of this effect increase with increasing amounts of nitrogen being introduced into the magnetic film. Therefore, we have developed a methodology for increasing the amount of nitrogen introduced into the magnetic film without sacrificing any parameters of the film, such as squareness, SNR, overwrite, and PW50.
In general, our research strongly suggests that low solubility elements may provide the exact benefits obtained by the introduction of nitrogen, namely the low noise provided by preferential segregation and destruction of intergranular exchange interaction. Therefore, in place of the nitrogen, other low solubility elements maybe employed, such as boron (B), phosphorous (P), sulfur (S), carbon (C), silicon (Si), arsenic (As), selenium (Se) and tellurium (Te).
The alloy of the present invention may also include additional elements such as tantalum (Ta), titanium (Ti), nickel (Ni), boron (B), Cr, etc. The concentrations of the additional elements of the alloy are chosen to maximize the magnetic and physical properties of the film as well as the incorporation of nitrogen. For example, CoNi0.07Pt0.12Ta0.015Ti0.015B0.02 (meaning a film made from a target having this composition) doped with at least 1 at. % nitrogen in the film has provided a high degree of squareness while either maintaining or improving Hc, SNR, OW, and PW50.
One method of forming the thin film magnetic alloy according to the present invention is sputtering in a nitrogen-rich ambient, so that the nitrogen is gettered in the sputtered film with the target elements. Another method is to form the thin film magnetic alloy by sputtering from a target which contains the alloy components doped with an appropriate amount of nitrogen.